DC-10 wind tunnel tests showed a significant loss in maximum lift coefficient in the flap deflected configurations, with landing slat extension, compared to predictions. This resulted in a stall speed increase of about 5 knots in the approach configuration. The initial wing stall occured behind the nacelles and forward of the inboard ailerons. The problem was traced by flow visualization techniques to the effects of the nacelle wake at high angles of attack and the absence of the slat in the vicinity of the nacelle pylons. The solution was developed in the NASA Ames Research Center 12 ft. pressurized tunnel and turned out to be a pair of strakes mounted forward on each side of the nacelles in planes about 45 degrees above the horizontal. The final strake shape was optimized in flight tests. The strakes are simply large vortex generators. The vortices mix the nacelle boundary layer air with the free stream and reduce the momentum loss in the wake. The vortices then pass just over the upper surface of the wing, continuing this mixing process. The counterrotating vortices also create a downwash over the wing region unprotected by the slat, further reducing the premature stall. The effect of the strakes is to reduce the required takeoff and landing field lengths by about 6%, a very large effect.

Quoting western727 (Thread starter):So, what about the 747-8 wing/engine design necessitated the departure from earlier models? Thanks in advance.

I suspect they are on the 747-8 for the same reasons you find them on the DC-10, MD11, A320 series, A340, A380, 737Classic (CFM56), 737NG, 767, and 777. The nacelle strakes/chines can result in significant reductions in flaps down stall speeds. Parameters influencing the effectiveness of these devices include nacelle diameter, local wing chord, wing sweep, gap between the wing and nacelle, leading edge device type, and the intersection of the deployed leading edge device with the nacelle strut.

If you look at the Airbus and Heritage Boeing porducts, you find only an inboard strake on the nacelles, rather than the pair found on the McD products.

[Edited 2010-02-13 07:58:03]

Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis

I always thought the strakes became a design requirement to ensure the engine fell DOWN AND AWAY from the wing in the event of a pylon failure after the ORD crash and the AMS cargo 747 crashes...I wasn't aware they had any effect on stall speed. Or was this simply a positive side-effect from adding them?

Quoting ANITIX87 (Reply 2):I always thought the strakes became a design requirement to ensure the engine fell DOWN AND AWAY from the wing in the event of a pylon failure after the ORD crash and the AMS cargo 747 crashes

Good point; I had thought the same until I read the other thread. What you and I thought makes sense...until one thinks about the mass of the engine assembly; the relative small size of the strakes therefore has little aerodynamic effect to force the engine down after separation. Further, the thrust of the engine while the engine assembly departs the wing (assuming a high commanded thrust level at the time of separation) virtually guarantees the engine will fly up and above the wing anyway when one thinks about it.

Further, after AA191 and the El Al AMS crash, classic 747s and 744s still lack strakes to this day.

EDIT: I also realized the prototype DC-10 HAD strakes installed and AA191's #1 engine still flew up and above the wing.

The GEnx-2B engines have a larger fan than previous 747s, as a result they had to be placed closer to the wing to get the required ground clearance like that of other 74s. The new airflow interaction of the engine and wing require strakes to be fitted to achieve the required performance like on other A/C which have engines positioned near to the wing.

Quoting CCA (Reply 4):The GEnx-2B engines have a larger fan than previous 747s, as a result they had to be placed closer to the wing to get the required ground clearance like that of other 74s

Makes perfect sense; I presume the 748 has the same basic landing gear design as the 744...at least as far as the height is concerned, unlike the 737NG which has taller MLG assemblies than the classics. Thanks, CCA.

Quoting CCA (Reply 4):The GEnx-2B engines have a larger fan than previous 747s, as a result they had to be placed closer to the wing to get the required ground clearance like that of other 74s. The new airflow interaction of the engine and wing require strakes to be fitted to achieve the required performance like on other A/C which have engines positioned near to the wing.

Also the 747-8 has a recontoured wing profile and an other type of trailing edge flaps installed. Dual inboard and single outboard slotted TE flaps, so the flow pattern under/over the wings and TE flaps has changed also.
In the wind tunnel the combination of the larger diameter engines and other wing and pylon changes let to the installation of additional strakes on the engine nacelles to correct the airflow pattern, with L.E flaps extended.

Operating a twin over the ocean, you're always one engine failure from a total emergency.